1. Technical Field
This invention relates to motion transmission devices and in particular to such devices to transmit motion from a first member to a second member via a magnetic coupling effect between them.
2. Background Art
Though not limited thereto the present invention is considered particularly useful for directly linking a rotating output shaft to a device which converts the rotational motion into linear or reciprocal motion. The most common known way of achieving this is to provide the rotatable shaft as or with a lead screw and have a follower unit travel longitudinally of the shaft along the threads of the lead screw.
In WO95/108860 the electrically induced rotation of an internally threaded, outer cylinder is transmitted to an internal, linearly driven shaft by means of rollers provided with rings which define camming surfaces engaged by the internally threaded bore of the cylinder and which engage annular rings to one end of the internal shaft. The drive transmission effected by the rollers is wholly mechanical in nature and the publication suggests that, to avoid slippage, the discs be provided with gear teethxe2x80x94a costly item. A similar arrangement, using a ball nut engaging a ball screw portion of a spindle, is disclosed in GB-2248976.
In WO96/19035 two mutually spaced rotary discs or cylinders have a rotary drive transmitted magnetically across the space between them. In one suggested embodiment the rotary discs are provided with a plurality of spaced magnets of disc-like form with opposite faces being North and South poles respectively. However, the angularly spaced nature of these magnets produces a step-wise motion transmission rather than a smooth motion transfer of substantially constant velocity.
GB-1333641 employs an electromagnet selectively to couple and de-couple magnetically a threaded nut and a threaded worm, the magnetic effect being across the gap between crests and troughs of the opposed threads. Like spacing between magnetised helical screw threads that are confrontingly arranged but are kept spaced apart are detailed in the linear-to-rotary converter (a so-called xe2x80x9charmonic drivexe2x80x9d) disclosed in GB-2088017. Another such xe2x80x9charmonic drivexe2x80x9dis disclosed in GB-2205452.
In EP-0583035, mutually spaced internal and external coupling members are each provided with a double helical thread and a magnetising member for generating a magnetic flux (in the form of a helical magnetic field) which permeates both coupling members, and follows a helical path between the double thread such that the individual threads form a pole shoe for the magnetising member. The magnetising member is provided by a series of axially polarised permanent magnets wholly embedded into the periphery of one of the coupling members. This arrangement is very expensive to produce commercially.
The present invention is concerned to overcome the complexity and manufacturing expense of motion transmission devices of the prior art.
According to one aspect of this invention there is provided a magnetic motion transmission device which comprises a magnetic body which has an outer cylindrical surface and radial end walls, is polarised axially of the cylindrical surface and, at the opposed radial ends of the cylindrical surface, has a pair of disc-like pole plates that are of a like diameter greater than the diameter of the cylindrical surface,
According to another aspect of this invention, to effect motion transmission between first and second members, there is provided apparatus including said members and means to provide a magnetic coupling effect between them, characterised in that
said first member comprises a device according to said first aspect of this invention, and in that
the second member has a surface engaged by said disc-like pole plates and is of magnetically permeable material at or at least adjacent to the engaged surface to contact said surface.
By effecting contact engagement between the first members pole pieces and said surface, the effect is created of a low reluctance path for the magnetic flux (in the manner of a keeper or of a pole shoe) between the disc-like pole plates.
Preferably the pole plates are mounted axially at the opposed radial ends of the magnetic body by an axle pin of non-magnetic material (e.g. brass). Optionally, for certain applications, the magnetic body may be provided with an axial bore providing a loose fit for the axle pin to permit the body to slew to a limited degree. If the said second member has a cylindrical or conical form, this loose-fitting axle pin permits the body to adopt a skewed position with respect to the axis of the cylinder or cone.
Advantageously, the said second member has a generally cylindrical or conical surface (with or without a helical formation therearound) and is rotatable about the axis of said surface. Advantageously the said first member is constrained to move longitudinally of that axis as it rotates about the said cylindrical or conical surface.
Preferably, a plurality (e.g. at least three) of said first members is provided, these members being mounted equi-angularly about the axis of said cylindrical or conical surface by cage means that allow them to rotate individually about their own axes.
Advantageously, where the surface of said second member is cylindrical, the plurality of first members may be encompassed by an outer ring member having an inner surface in engagement with the disc-like pole pieces and, at least adjacent to the engagement surface, comprising a magnetically permeable material to provide the effect of a low reluctance path for the magnetic flux (in the manner of a pole shoe) between the disc-like pole plates.
Preferably the said second member has a generally cylindrical surface and the latter is provided with a helical formation, e.g. a screw thread or spiral spring provided thereon, that is engaged by the disc-like pole pieces. In use, as the second member is rotated about the axis of its generally cylindrical surface (and helical formation), the pole pieces constrain the or each said first member to move generally axially thereof around its own individual axis andxe2x80x94by virtue of the magnetic coupling between said membersxe2x80x94as a whole with respect to the surface of said second member.
Where the device incorporates the preferred features of the last two paragraphs, and the outer ring member is constrained against rotatary motion, the outer ring member will be moved axially by the axially moving first member(s) but such motion will be at a reduced rate since the (or each) first member rotates in an idling modexe2x80x94whilst maintaining contact of its pole pieces with both the outer ring and the said second member. This measure of idling is primarily related to the diameter of the inner surface of the outer ring member, the diameter of the disc-like pole pieces, the diameter of the generally cylindrical surface of the second member and the ratios of one to the other.